When considering flow processes in rheometers, but also in practical components such as dies, plasticizing units, and between roll gaps, it is usually assumed that the flowing melt adheres to the wall. In the case of polymer melts, experience shows that this applies to the majority of materials. However, there are some materials, including high-viscosity polyethylenes or polyvinyl chloride (PVC, often provided with external lubricants for gentler processing), which tend to exhibit wall-slip effects at high shear stresses or high shear rates.

When the transition from wall adhesion to wall slip occurs, most equations used to calculate a flow are no longer valid. Since this is a very complex phenomenon that depends on viscosity, shear rate, shear stress, temperature, and pressure, a simple and stable empirical model is used.

Due to the complex relationships involved, the influence on the pressure-throughput behavior is neglected; instead, only the reduction in shear dissipation is taken into account in the temperature calculation.

In REX/PSI, wall slip occurs above a critical shear stress. This can be specified as a linear equation as a function of temperature. REX/PSI therefore requires the input of two pairs of values for the critical shear stress and the corresponding temperature.

For the dissipation calculation, the following applies:

$$ \tau_0 = \eta_0 \cdot \dot{\gamma} $$ $$ if $$ $$ \tau_0 > \tau_{crit}(T) $$ $$ then $$ $$ \Delta \tau = \tau_0 - \tau_{crit}(T) $$ $$ \tau_{target} = \tau_{crit}(T) + \Delta \tau \cdot k_{mat} $$ $$ with $$ $$ 0 < k_{mat} \leq 1 $$ $$ it~follows~that $$ $$ \eta_{wall~slipping} = \frac{\tau_{target}}{\dot{\gamma}} $$

If the local shear stress is greater than the critical shear stress, the shear stress is reduced by means of the factor $k_{mat}$​. A value of $0$ corresponds to a hard limit at $\tau_{krit}$​, while a value of $1$ would mean no wall slip. Due to the reduced shear stress, a reduced representative viscosity can in turn be calculated, which is used for calculating dissipation. Wall slip therefore leads to a reduction in temperature.

Important:

If wall slip is checked in the material data, the values entered there are used. If wall slip is unchecked there, wall slip is still calculated using predefined values, since experience has shown that, especially for high-viscosity polymers with correspondingly high shear stresses, an excessively high temperature would otherwise be calculated. The default values used in this case are a critical shear stress of $\tau_{krit}=100 kPa$ and $k_{mat}=0,25$